This invention pertains to methods for manufacturing magnetic disks comprising carbon protective overcoats and the resulting magnetic disks.
FIG. 1 illustrates in cross section a magnetic disk 10 in a disk drive 12. Magnetic disk 10 comprises a substrate 14 (e.g. glass, glass ceramic, or NiP-plated aluminum), an underlayer 16 (e.g. Cr, a Cr alloy, NiP, NiAl or other appropriate material), a magnetic layer 18 (e.g. a Co alloy), and a protective overcoat 20 (e.g. hydrogen-doped carbon, nitrogen-doped carbon, or carbon doped with both hydrogen and nitrogen). A lubricant layer 22 (e.g. perfluoropolyether) is applied to protective overcoat 20.
Magnetic disk 10 is mounted on a spindle that is rotated by a motor 24. A read-write head 26, mounted on a suspension 28, xe2x80x9cfliesxe2x80x9d above the rotating disk. Head 26 comprises a slider including a hard Al2O3xe2x80x94TiC body 30 with a read-write element 32 formed on the trailing edge thereof. A carbon overcoat 34 is formed on the bottom surface (the air bearing surface) of head 26 for tribological purposes.
Magnetic layer 18 performs the function of storing data. Overcoat 20 performs several functions:
a) It prevents corrosion of magnetic layer 18.
b) It is hard, and prevents mechanical damage of magnetic layer 18.
c) It exhibits low static and dynamic friction.
d) It holds lubricant layer 22 on disk 10.
e) It prevents wear of disk 10.
Industry has devoted a large amount of time and effort trying to form appropriate carbon films to be deposited on magnetic disks as protective layers. For example, F. K. King, xe2x80x9cDatapoint Thin Film Mediaxe2x80x9d, IEEE Trans. Magn., July 1982, discuses sputtering carbon on a magnetic disk. U.S. Pat. No. 5,045,165, issued to Yamashita, discusses sputtering a hydrogen-doped carbon film on a magnetic disk to prevent wear and corrosion. Yamashita teaches that the hydrogen enhances wear resistance of the carbon. European Patent Application EP 0 547 820 discusses sputtering a nitrogen-doped carbon film on a magnetic disk. The ""820 application states that the nitrogen reduces stress in the carbon, and reduces the likelihood that the carbon will delaminate from the disk. U.S. Pat. No. 5,837,357 discusses a magnetic disk comprising a hydrogen-doped carbon film covered by a nitrogen-doped carbon film. U.S. Pat. No. 5,232,570 also discusses sputtering carbon on the magnetic disk in the presence of nitrogen. Other references pertaining to carbon overcoats for magnetic disks include U.S. Pat. No. 5,855,746 and PCT Patent Application WO 99/03099. This list is by no means exhaustive.
Protective carbon overcoats for magnetic disks are typically formed by sputtering. Because of the way in which they are formed, they comprise mostly SP2 carbon. Industry has been using such carbon films for many years, and has considerable experience with these films. Thus, various types of lubricants have been developed which can be applied to predominantly SP2 carbon films to cause these films to exhibit low friction and stiction. (As used herein, the term xe2x80x9cpredominantly SP2 carbonxe2x80x9d means that of the carbon bonds in the film, more of those bonds are SP2 than any other type of bond. Similarly, xe2x80x9cpredominantly SP3 carbonxe2x80x9d means that of the carbon bonds in the film, more are SP3 than any other type of bond.)
Recently, Komag (the assignee of the present invention) developed a new type of carbon overcoat comprising more than 70% SP3 carbon. This type of carbon overcoat is described by Wen Hong Liu et al. in U.S. patent application Ser. No. 09/298,107 now U.S. Pat. No. 6,086,730, filed on Apr. 22, 1999, incorporated herein by reference. The ""107 carbon is deposited by applying a novel voltage waveform to carbon sputtering targets. It has been discovered that this carbon overcoat is extremely hard and scratch resistant.
There are other types of carbon overcoats that have high SP3 contents. In particular, one can form a carbon film using chemical vapor deposition, ion beam deposition, or cathodic arc deposition. Weiler et al., xe2x80x9cDeposition of Tetrahedral Hydrogenated Amorphous Carbon Using a Novel Electron Cyclotron Wave Resonance Reactorxe2x80x9d, Applied Physics Letters, Vol. 72, No. 11, Mar. 16, 1998, discusses ion beam deposition of carbon. Kang, et al., xe2x80x9cEvaluation of the Ion Bombardment Energy for Growing Diamondlike Carbon in an Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Depositionxe2x80x9d, J. Vac. Sci. Technol. A. 16(4), July/August 1998, discusses using chemical vapor deposition to form a carbon film. J. Robertson, xe2x80x9cUltrathin Carbon Overcoats for Magnetic Storage Technologyxe2x80x9d, TRIB-Vol. 9, Proceedings of the Symposium on Interface Technology Towards 100 Gbit/in2, ASME 1999 discusses cathodic arc deposition. Other references include U.S. Pat. No. 5,476,691; Brown, xe2x80x9cVacuum Arc Ion Sourcesxe2x80x9d, Rev. Sci. Instrum. 65(10), October 1994, Sanders, et al., xe2x80x9cCoating Technology Based on the Vacuum Arcxe2x80x94a Reviewxe2x80x9d, IEEE Transactions on Plasma Science, Vol. 18, No. 6, 1990; and Anders et al., Mechanical Properties of Amorphous Hard Carbon Films Prepared by Cathodic Arc Depositionxe2x80x9d, Mat. Res. Soc. Symp. Proc. Vol. 383, 1995. Japanese laid-open publication 62-183022 discusses using a plasma CVD process to make a carbon film on a magnetic disk. Weiler, Kang, Robertson, the ""691 patent, Brown, Sanders, Anders, and the 62-183022 references are incorporated herein by reference.
SP3 carbon has an atomic structure that differs from SP2 carbon. Accordingly, the behavior of SP2 carbon can be quite different from SP3 carbonxe2x80x94sometimes to an unpredictably great extent.
As mentioned above, magnetic disk drive 12 contains magnetic disk 10 with carbon protective overcoat 22 and lubricant 24 applied to the disk. The disk substrate 14 is textured to minimize friction and stiction between disk 12 and read-write head 26. The disk/read-write head interface constitutes a finely tuned tribological system designed to minimize static and dynamic friction and wear. The texturing of the disk, the composition, deposition conditions and structure of carbon protective overcoats 22 and 34, the other elements added to the carbon overcoats, the types of lubricants, the additives in the lubricants, lubricant application process and related parameters are determined based on exhaustive research to ensure that the disk drive can survive a large number of on/off (contact-start-stop, or xe2x80x9cCSSxe2x80x9d) cycles. Changing one element in this tribological system can alter the behavior of the entire system. For example, if one were to replace a conventional type of predominantly SP2 carbon with a different type of carbon, e.g. a predominantly SP3 carbon, that can completely change the behavior of the tribological system.
Merely by way of example, it has been discovered that when one tries to use the ""107 type carbon and a perflouropolyether lubricant such as Z-dol (manufactured by Montedison Co. of Italy) mixed with an X1P additive, for reasons not well understood, the resulting disks tend to fail glide tests. This is particularly interesting and unexpected, since the lubricant thickness is only about 3 nm, whereas the glide testing is performed at a glide height of about 1 microinch, or about 25 nm. Thus, it is highly unexpected that the lubricant could interact with the carbon film in such a way as to cause a failure in a glide test where the glide height is eight times the lubricant thickness.
Certain forms of high SP3 carbon formed by chemical vapor deposition have been found to exhibit other problems, i.e. sensitivity to certain types of contaminants.
A method in accordance with the invention comprises depositing first and second carbon layers on a magnetic disk and then applying a lubricant to the magnetic disk. In one embodiment, the first carbon layer is predominantly SP3 carbon. The first layer can have 70% or greater SP3 bonding. The second layer comprises less than or equal to 50% SP3 bonding. The second layer can be extremely thin, e.g. a flash layer of having a thickness between 0.1 and 1 nm. The lubricant can be a perfluoropolyether lubricant.
Of importance, the high SP3 content protective layer is extremely hard, and resists wear and scratching. Because the second protective layer is so thin, it does not add substantially to the separation of the magnetic film within the disk and the read-write head.
It has further been discovered that although the second protective layer is extremely thin, the properties of the second protective layer control the manner in which the lubricant cooperates with the disk. In particular, although the second carbon layer is only 0.1 to 1 nm thick, the lubricant bonds with, and adheres to the second carbon layer in the same way that the lubricant would cooperate with the carbon on a conventional magnetic disk. The second carbon layer can mask any deleterious effects that the high SP3 content of the first carbon layer would otherwise have on the disk""s interaction with the lubricant.
As mentioned above, the first and second carbon layers have different structures. Because the first carbon layer has mostly SP3 bonds, it has a density greater than about 2.1 grams/cc, and typically about 2.5 grams/cc. In contrast, the second carbon layer has a lower density, e.g. less than about 2.1 grams/cc, and typically 1.8 grams/cc.
The first carbon layer has a refractive index that is greater than 2.0, and typically about 2.1. The second carbon layer has a lower refractive index than the first carbon layer, less than about 2.0 and typically about 1.8.
In one embodiment, the first carbon layer has a lower surface energy than the second carbon layer. (One way of measuring the surface energy is by a water contact energy test. The difference in water contact angle between the first and second carbon layers can be greater than 3 degrees and in one embodiment, greater than about 5 degrees. This difference in water contact angle is typically less than about 8 degrees.)
In accordance with another aspect of the invention, a new type of carbon overcoat is introduced into the manufacturing process for a magnetic disk without requiring the exhaustive optimization and reengineering that normally occurs when one makes a change to one of the elements of the tribological system of the disk and read-write head. In accordance with this aspect of the invention, a process for manufacturing a magnetic disk initially comprises the steps of:
a) providing a structure comprising a substrate with a magnetic layer thereon;
b) depositing a first carbon overcoat on the magnetic layer (e.g. a predominantly SP2 carbon overcoat formed by sputtering); and
c) applying a lubricant layer on the protective overcoat. A method in accordance with the invention comprises replacing the step of depositing the first carbon overcoat with the step of providing a carbon overcoat having characteristics that are different from those of the first overcoat (e.g. an overcoat with predominantly SP3 carbon), followed by the step of a depositing a very thin layer of carbon using the same or substantially the same deposition conditions as those used to form the first carbon overcoat. For example, the process gas composition, pressure, and flow rate are the same or substantially the same. The substrate bias and temperature can be the same or substantially the same. Thus, the top surface of the magnetic disk, comprising the very thin layer of carbon, cooperates with the lubricant in substantially the same way as the above-mentioned first layer of carbon. Therefore, it is not necessary to do the substantial testing and engineering work that would otherwise need to be done if one simply replaced the first carbon overcoat with a predominantly SP3 carbon overcoat.
The predominantly SP3 carbon overcoat can be formed using the method of the ""107 patent, or it can be deposited by CVD or cathodic arc deposition.